Grating based plasmonic band gap cavities

Cataloged from PDF version of article.We report on a comparative study of grating based plasmonic band gap cavities. Numerically, we calculate the quality factors of the cavities based on three types of grating surfaces; uniform, biharmonic and Moiré surfaces. We show that for biharmonic band gap cavities, the radiation loss can be suppressed by removing the additional grating component in the cavity region. Due to the gradual change of the surface profile in the cavity region, Moiré type surfaces support cavity modes with higher quality factors. Experimentally, we demonstrate the existence of plasmonic cavities based on uniform gratings. Effective index perturbation and cavity geometries are obtained by additional dielectric loading. Quality factor of 85 is obtained from the measured band structure of the cavity. © 2009 Optical Society of America

Radiation Impedance of an Array of Circular Capacitive Micromachined Ultrasonic Transducers

Cataloged from PDF version of article.The radiation impedance of a capacitive micromachined ultrasonic transducer (cMUT) with a circular membrane is calculated analytically using its velocity profile for the frequencies up to its parallel resonance frequency for both the immersion and the airborne applications. The results are verified by finite element simulations. The work is extended to calculate the radiation impedance of an array of cMUT cells positioned in a hexagonal pattern. A higher radiation resistance improves the bandwidth as well as the efficiency of the cMUT. The radiation resistance is determined to be a strong function of the cell spacing. It is shown that a center-to-center cell spacing of 1.25 wavelengths maximizes the radiation resistance, if the membranes are not too thin. It is also found that excitation of nonsymmetric modes may reduce the radiation resistance in immersion applications

Interaction between a cMUT cell and a liquid medium around the parallel resonance frequency

In this paper, we present how a capacitive micromachined ultrasonic transducer (cMUT) couples to the immersion medium, based on an accurate parametric model. We show that the velocity of cMUT membrane can be written as a sum of an average velocity term and a residual term. We demonstrate that this residual term carries non-zero energy at the parallel resonance frequency by investigating the interaction between the cMUT cell and a liquid medium. We develop a model that is also applicable around the parallel resonance frequency. © 2007 IEEE

Plasmonic band gap cavities on biharmonic gratings

In this paper, we have experimentally demonstrated the formation of plasmonic band gap cavities in infrared and visible wavelength range. The cavity structure is based on a biharmonic metallic grating with selective high dielectric loading. A uniform metallic grating structure enables strong surface plasmon polariton (SPP) excitation and a superimposed second harmonic component forms a band gap for the propagating SPPs. We show that a high dielectric superstructure can dramatically perturb the optical properties of SPPs and enables the control of the plasmonic band gap structure. Selective patterning of the high index superstructure results in an index contrast in and outside the patterned region that forms a cavity. This allows us to excite the SPPs that localize inside the cavity at specific wavelengths, satisfying the cavity resonance condition. Experimentally, we observe the formation of a localized state in the band gap and measure the dispersion diagram. Quality factors as high as 37 have been observed in the infrared wavelength. The simplicity of the fabrication and the method of testing make this approach attractive for applications requiring localization of propagating SPPs. © 2008 The American Physical Society

Improved performance of cMUT with nonuniform membranes

When capacitive micromachined ultrasonic transducers are immersed in water, the bandwidth of the device is limited by the membrane's second resonance frequency. At this frequency no mechanical power to immersion medium can be transferred. We present a membrane shape to shift the second resonance frequency to a higher value. The structure consists of a very thin membrane at the outer rim with a rigid mass at the center. The stiffness of the central region moves the second resonance to a higher frequency. This membrane configuration is shown to work better in terms of gain and bandwidth as compared to conventional uniform membranes in both transmission and reception. © 2005 IEEE

Calculation of transformer ratio in mason's equivalent circuit for cMUTs

We present a new method to calculate the transformer ratio of a cMUT in Mason's Equivalent circuit model. The effect of the spring softening capacitance is also included in the analysis. We use the existing turns ratio calculation methods as a starting point to calculate the force-voltage ratio at the secondary of the transformer and the input port of the circuit. We use this ratio and the capacitances in the Mason's circuit to find the actual turns ratio. Different methods are discussed for the calculation of the equivalent circuit parameters. We show that the transformer ratio has a bounded maximum at collapse voltage. We also investigate the effect of electrode size on the transformer ratio. Transformer ratio decreases with decreasing electrode size. © 2006 IEEE

Stagger tuned cMUT array for wideband airborne applications

In this study, we explore the limits of cMUTs in air-borne applications. First we investigate the ways of increasing the bandwidth of a single cMUT cell in air. The effect of array operation is also considered in order to increase the radiation resistance seen by the transducer. We calculate the bandwidth of a stagger tuned cMUT array. It is shown in this paper that more than 60% bandwidth can be obtained by three staggered frequencies. © 2006 IEEE

Bandwidth improvement in a cMUT array with mixed sized elements

A capacitive micromachined ultrasonic transducer (cMUT) is typically fabricated by concatenation of several cMUT cells with identical physical dimensions. If the membrane thickness is kept fixed, the radius of the cMUT determines the center frequency of operation. A smaller radius implies a greater center frequency. Therefore, it should be possible to put cMUTs with different sizes in parallel to get a larger bandwidth at the expense of gain. In this study, we investigate the optimization of the bandwidth characteristics of a cMUT by using mixed size cells. We designed two mixed size cMUT arrays with a predicted optimized fractional bandwidth value of about 155% at 5.4 MHz, and 146% at 8.8 MHz. These values are about 55% and 58% better than what can be achieved with a uniform size array at the corresponding center frequencies. There is almost no loss in the gain bandwidth product when two different sized cMUTs are used in parallel. There is about 9% increase in gain bandwidth product when three different sized cMUTs are used in parallel. It is shown, in this study, that gain bandwidth product and bandwidth can be enhanced by use of mixed size cMUT cells. © 2005 IEEE